In a nuclear power generation facility, a nuclear reactor core (a/k/a, reactor vessel) contains nuclear fuel rods, and is equipped to initiate, control, and sustain nuclear chain reaction in the nuclear fuel rods to generate heat. The heat generated by the nuclear reaction is absorbed by a circulating primary coolant into which the fuel rods are immersed, maintaining a stable operating temperature. The circulating coolant limits the operating temperature and thus keeps the nuclear reaction in control; it also carries away the heat generated by the controlled nuclear reaction which is in turn used to produce pressurized steam that drives a turbine. The turbine in turn drives a power generator to produce electricity. The most common types of nuclear reactors use closed-loop circulating purified water as the primary coolant, which in Boiling Water Reactors is boiled into steam by the nuclear reaction to drive a turbine, and then condensed back into liquid phase to be cooled with separate cycling cooling water drawn from a large body of external supply, such as a sea, a river, or a lake. An older version of nuclear power reactor design, the Pressured Water Reactor (PWR) further separates the reactor vessel coolant from the heated water that generates steam to drive a turbine.
The nuclear fuel material is contained in a tube-like rod made with radiation-neutral zirconium alloy. Such an assembly is called a fuel rod. During the reactor operation in the power generating mode, the surface temperature of the fuel rod cladding is normally kept at approximately 280 degrees Celsius. The nuclear reaction is further moderated and controlled by control rods inserted between the fuel rods to absorb neutrons generated by nuclear reactions in the fuel rods. The zirconium alloy is transparent to neutrons, which is the reason it is used as the cladding material for the nuclear fuel. Unfortunately, when heated to 550 degrees Celsius or above, zirconium reacts with steam and generates hydrogen, which is highly explosive at high temperature and the presence of oxygen. Explosions in and around a nuclear reactor in such a situation are certain to spew extremely dangerous radioactive material into the environment. Furthermore, the disintegration of the fuel cladding causes the nuclear fuel material to fall to the bottom of the reactor vessel to continue the out of control nuclear reaction and the continued elevation of temperature, which is called a melt-down.
Even when the reactor is shut down and the stimulated chain reaction is stopped, the nuclear fuel will continue its intrinsic decay and reaction, with the generated heat spontaneously increasing the rate of reaction until the remaining reactive material is entirely spent. Cooling the nuclear material therefore is critical to keep the reaction under control and below a threshold rate that can cause spontaneous acceleration. Above that threshold the spontaneous acceleration of reactive processes will lead to out-of-control conditions which may result in harmful radiation with the accompanying radioactive by-products to be released into the environment. Therefore, whether the fuel rods remain in the reactor vessel, or are kept in storage outside the reactor vessel, the continual cooling of the fuel rods is required at all times. Even spent nuclear fuel is typically stored in cold water pools and needs to be continually cooled for several years before the spent fuel rods can be safely removed to dry and permanent storage. When an accident, equipment malfunction, loss of power, or operator error causes the reactor to lose cooling, which is conventionally facilitated only by electrically powered pumps circulating the primary and secondary coolants immersing the fuel rods, the fuel rods will rapidly heat up resulting in a self-propelling cycle of increased heating and accelerated nuclear activity, soon reaching the critical temperature of 550 degrees Celsius and higher, where the zirconium cladding will react with steam. In the presence of water vapor in the vicinity of the fuel rods, the zirconium and steam react to immediately generate copious amounts of explosive hot hydrogen gas. When brought into contact with any form of oxygen in the environment, disastrous explosions result until the zirconium and the nuclear reactive material are both exhausted, the environment totally destroyed, or safe cooling is installed and the temperature is brought under control.Zr+4H2O@˜550C=Zr(OH)4+4H2 
In addition, when zirconium alloy casing disintegrates during its reaction with steam, it allows the nuclear fuel pallets to drop to the bottom of the steel reactor vessel, out of reach of all other conventional nuclear activity control mechanisms that may still be functioning. The fuel temperature in that case would continue to rise even more rapidly until the fuel melts and forms a pool at the bottom of the reactor vessel which can burn through the vessel wall into the floor of containment chamber, and even melt through the containment chamber floor, and expose the molten nuclear fuel, its continual nuclear reaction, and massive radioactive by-products into the environment. This is called a nuclear meltdown.
In the case of the recent Fukushima Nuclear Reactor crisis in Japan, the electrically powered cooling system failed through the earthquake, and the backup electrical power generators failed due to the tsunami flooding. The fuel rods in the six reactors and the cold water pools that store the spent fuel lost cooling.
After the tsunami passed, the Japanese Government and TEPCO (Tokyo Electric Power Corporation) operators used portable generators and pumps to pump sea water into the power plants and reactor vessel to cool the overheated fuel rods. Concerned that the sea water is highly corrosive, under Japanese Government's request for assistance, US ships shipped a vast amount of purified water to the Fukushima site for cooling the reactor vessels. Unfortunately, the vast amount of steam thus generated, whether from sea water or from purified water, interacted with the overheated zirconium cladding of the fuel rods (at and above 550 degree C.) and produced copious amounts of hydrogen gas, resulting in repeated explosions.